A semiconductor injection laser typically comprises a body of semiconductor material having a thin active region between cladding regions of opposite conductivity type. To increase the output power from such a laser, a guide layer having a refractive index which is intermediate between that of the active and cladding layers is interposed between one of the cladding regions and the active region. Light generated in the active layer propagates in both the active and guide layers thereby forming a larger beam at the emitting facet of the body. A thin active layer restricts oscillation in the transverse direction, the direction perpendicular to the plane of the layers, to the fundamental optical mode. In the lateral direction, the direction in the plane of the layers and perpendicular to a line between the laser facets, a similar restriction does not exist and oscillation typically occurs simultaneously in several different optical modes.
It has been found useful to introduce lateral variations into the laser structure which produce an optical waveguide which restricts the oscillation to the fundamental optical mode in the lateral direction. A channelled substrate laser formed by liquid phase epitaxy over a single channel in a substrate has an optical waveguide formed by lateral variations in the layer thicknesses and the close proximity of the absorbing substrate at the sides of the emitting region over the channels. The lateral flow of electrical current is not, however, automatically restricted to the emitting region over the channel but rather tends to flow to the substrate at the sides of the channel. To inhibit this lateral current flow, back-biased P-N junctions are typically formed in the substrate or overlying layers at the sides of the channels. Botez in U.S. Pat. No. 4,347,486, incorporated herein by reference, has disclosed a laser having a pair of channels in the surface of a substrate with a mesa therebetween. The layers overlying this channelled surface have laterally varying thicknesses because of the tendency for faster liquid-phase epitaxy growth over concave as opposed to flat or convex surfaces. This structure restricts the current flow to the region over the mesa and, because of the laterally varying layer thicknesses, produces an optical waveguide which restricts the oscillation to the fundamental lateral mode over the mesa up to an output power in excess of 40 milliwatts.
To increase the output power in the coherent light beam beyond the capability of an individual laser, monolithic arrays of spaced-apart laser devices have been fabricated where the modes of oscillation of the individual lasers are coupled to one another to form a single phase-locked coupled oscillator. Such lasers include a striped-oxide defined array having planar layers over a planar substrate which operates only in a pulsed mode and an array of mesa waveguide lasers where the emitting regions are over mesas on the substrate surface. This array appears to operate in a phase-locked mode in pulsed operation but is only partially phase-locked in continuous wave operation. Botez in U.S. Pat. No. 4,385,389 has disclosed a phase-locked array comprising a plurality of spaced-apart lasing elements of the type disclosed in U.S. Pat. No. 4,347,486, which can be operated cw in a fundamental lateral mode. In this array coupling between the modes of oscillation of the different elements of the array can occur over comparatively long distances because the individual devices have high lateral radiation leakage. However, the large inter-element spacing of the Botez array, required by the use of pairs of channels and the curvature of the layers, is undesirable since it increases the number of lobes in the far-field pattern. Thus it would be desirable to have a phase-locked laser array having the minimum spacing between the emitting elements and which operates in a single narrow beam peaked at 0.degree..